I have been interested in the use of biochar for a long time, and over decade ago I was
doing a fair amount of research for an independent think tank into the possibilities of using
this soil amendment.
It was fascination to delve into the details, processes, and potential uses for biochar,
especially at a time when we were only beginning to understand the possibilities.
At about the same time, I was also starting to take food growing much more seriously,
and I always wanted to explore the use of biochar in my gardens.
But I didn’t do any serious explorations at that time, partially because I wasn’t
sure how I was going to make it, and always felt that I would get back to it when I had
a chance.
This winter I finally began producing biochar for use in some of my gardens in this coming growing season.
Despite the diverse possible benefits of the use of biochar, it's essentially a simple thing.
Wood or other organic material is burnt without enough oxygen, producing what is essentially charcoal.
It is then charged with fertility and buried in soil, where it can potentially increase
the nutrient holding capacity of that soil.
The lasting beneficial effects have been seen in the highly fertile manmade terra preta
or black soils that were discovered in the amazon basin.
The charring of material allows the carbon to persist in the soil much longer than other
forms of organic matter typically can.
In addition the open structure of the microscopic pores of the biochar can be a home to diverse
soil biology, and it has the ability to hold onto nutrients in the soil, in a manner that
is similar to humus.
The biggest issue with biochar is how to make it, or at least that's been one of the biggest
sticking points for me There are of course traditional methods for
making charcoal, including mounds or pits of wood covered with soil, though these tend
to smoulder and can release a lot of pollutants.
There have been a diverse range of purpose built retorts or burners designed, from very
simple to complicated, and I always thought I’d need to make or buy one of these in
order to produce biochar.
A few years ago I came across a cone method for burning charcoal, which seemed to be an
ingeniously simple method.
And then from that innovation, others developed the pit cone method, which only required digging
a hole in the ground.
This simple and effective low tech option seemed to be a great place to start, and the
only thing stopping me from making biochar was collecting the material and finding the
time to give it a try.
I collected a lot of woody material last winter, including a lot of branches from a beech tree
that blew down in remenants of Hurricane Ophelia.
There was also a lot of volunteer willow trees growing in scrap ground that needed to be
cut back, as well as pruning from the many apple trees on site.
This was all fairly low value material, with not a lot of other uses, and I spent the time
to collect it and bundle this material up, and then I stored it and tried to keep it dry.
Then this winter I dug a cone shaped pit in the ground, and started a fire in the bottom
of it.
As the fire burned, I added another layer of wood or twigs, keeping an eye out for when
white ash was starting to form on the surface of the sticks.
This ash indicated that the carbon base of the wood was now starting to burn, which was
something that I wanted to prevent.
So I kept slowly building up the fire, adding layer after layer of wood to the fire until
it reached the top of the cone.
Then I doused it with water to put out the fire and kept adding more water until I thought
it was cold enough.
This last step is what starts to separate biochar from charcoal, to make traditional
charcoal I would need to use some other method to stop the burning.
I then chopped up this char in a pail, put it through a sieve to remove any of the unburnt
pieces and crushed the larger pieces of char to what I thought were more appropriate sizes.
Adding a new layer of material once the ash starts to form is the ingeniously simple part
that makes the whole thing work.
Once this fresh material starts to burn, it uses up all the oxygen and prevents oxygen
from reaching the already burning material lower in the cone.
But because there is still a lot of heat, all the burnable gasses and smoke continue
to be released from the wood underneath, but it has nowhere to go but up through the flames
above to be burnt.
When the freshly added material covers the whole surface and catches fire, a cone of
flames spreads out around the entire ring of the fire, and produces a virtually smokeless burn.
But this inverted cone of flames often has a dark centre, where oxygen can’t get to,
but the gasses and the smoke is burnt off in the envelope of flames, it is quite a beautiful
to watch.
I have found that the wind doesn’t help with this, especially if it's gusty, as it
can increase the amount is smoke that is produced, but when the wind isn’t present, the convection
of the fire draws everything into the centre to be burnt.
After a relatively slow start to the fire, the amount of material that is added in each
layer increases exponentially, as the surface area of the fire increases, as well as the
overall heat, and in the end of the process you can burn lot of material fairly quickly.
As with many tasks in the gardens there are a few techniques that make things work better.
In the case of the pit char method, knowing when to add the next layer, and how much to
add at any one time, is perhaps the most significant skill involved.
Leave it too late and it may take too long to get the material to catch fire, causing
a lot of smoke and allowing more of the char underneath to be burnt off to produce ash.
Adding too much at any one time can smother the flames, and adding to early risks a much
bigger fire and perhaps a few singed eyebrows.
It seems to be a process of finding a balance between the ease of managing, the amount of
smoke produced, and the speed that the whole process can take.
Although it can be fairly time consuming, it can be a great way to make use of low value
woody material and tree prunings.
I tried burning some freshly pruned green branches, and some wood that had become a
bit damp, and seems possible to use them, although it produces a much slower fire and
is harder to avoid the smoke.
But burning green material could eliminate the need to store large volumes of material
while it dries, and I think that mixing some green material in with properly dried material
may be a good balance.
Dousing the fire takes lot of water and I found that I needed to make sure the fire
was out to the full depth, as had one batch that continued to burn and turned to ash after
I left it, having thought that it was cold enough.
Much of this is easy to figure out by trial and error, but the one thing that I’m really
not sure about is how fine the char needs to be crushed, as I have heard different recommendations.
And I haven’t figured out a really easy and effective way to crush it yet, so for
now I’m not too concerned with getting everything really small.
Now that I’ve burnt a lot of charcoal, I need to inoculate or charge it properly, to
fill it with fertility, as adding empty charcoal to the soil can apparently lock up a lot of
nutrients from within the soil.
I plan to mix it with compost, and some extra fertility, and to let it mellow for a while,
and there seem to be lots of options for this critical step that fully transforms the material
from charcoal to biochar.
Then I'll mix it into the soil of the garden bed, and hope that it does actually significantly
boost fertility holding capacity of soil and positively impact soil biology as well.
Otherwise it may not be worth the effort of collecting, burning and crushing all of this material.
I plan to do a few pot trials to see what effect, if any it has, with different concentrations
of it in the soil, and different methods and ingredients for preparing the biochar.
It should be really interesting to see what effect it has, and there seems to be so much
more to explore with the use of biochar.
But I’m really glad that I'm finally starting to produce this famous black soil amendment,
as it's something I’ve wanted to do for ages.
This is the first video I’ve made about biochar, but I’m planning to make more in
the future.
I’ll definitely produce a video showing the results of any trials that I make, and
I’m also looking at producing another video that goes into more detail about the methods
and possibilities of making biochar, and inoculating or charging it with different materials.
I’m also interested in exploring the possibilities and viabilities of using biochar as a form
of carbon sequestration, especially at a small scale.
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